Data storage devices including, e.g., those normally provided as part of, or in connection with, a computer or other electronic device, can be of various types. In one general category, data is stored on a fixed or rotating (or otherwise movable) data storage medium and a read head, a write head and/or a read/write head is positioned adjacent desired locations of the medium for writing data thereto or reading data therefrom. One common example of a data storage device of this type is a disk drive (often called a “hard” disk or “fixed” disk drive).
Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks, divided into sectors. Information is written to and read from a disk by a head (or transducer), which is mounted on an actuator arm capable of moving the head along a (typically arcuate) path to various radial positions over the disk. Accordingly, the movement of the actuator arm allows the head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the head to access different sectors on the disk. The head may include separate or integrated read and write elements.
A disk drive 10, exemplary of numerous types of drives that can be used in connection with embodiments of the present invention, is illustrated in FIG. 1. The disk drive comprises a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. The disk drive 10 also includes an actuator arm assembly 18 having a head 20 (or transducer) mounted to a flexure arm 22, which is attached to an actuator arm 24 that can rotate about a bearing assembly 26 that is attached to the base plate 16. The actuator arm 24 cooperates with a voice coil motor 28 in order to move the head 20 relative to the disk 12. The spin motor 14, voice coil motor 28 and head 20 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device. Instead of a one-disk and, therefore, a plurality of corresponding actuator arm assemblies 18.
FIG. 5 is a diagrammatic representation of a simplified top view of a disk 512 having a surface 542 which has been formatted to be used in conjunction with a sectored servo system (also known as an embedded servo system). As illustrated in FIG. 5, the disk 512 includes a plurality of concentric tracks 544a–544h for storing data on the disk's surface 542. Although FIG. 5 only shows a relatively small number of tracks (i.e., 8) for ease of illustration, it should be appreciated that typically tens of thousands of tracks are included on the surface 542 of a disk 512.
Each track 544a–544h is divided into a plurality of data sectors 546 and a plurality of servo sectors 548. The servo sectors 548 in each track are radially aligned with servo sectors 548 in other tracks, thereby forming servo wedges 550 which typically extend radially across the disk 512 (e.g., from the disk's inner diameter 552 to its outer diameter 554).
One of the operations that a disk drive performs is known as a seek operation. During a seek operation, the head 20 is moved from a present track of the disk to a target track of the disk, so that a data transfer can be performed to or from the target track. In order for a seek operation to be performed, a current is delivered to the voice coil motor (VCM) 28 of the disk drive, which causes the actuator arm 24 to rotate, thereby moving the head 20 along an are intersecting various radial positions relative to the disk surface 542.
With reference now to FIG. 2, a diagrammatic representation of a sectional view of a disk 12 and a head 20 is illustrated. As shown in FIG. 2, during operation, the head 20 (which, as illustrated, includes a slider) is located above disk surface 42 by a spacing 100 known as the flying height of the head 20. The spacing 100 is created by the interaction between air currents above the surface of the disk 12 (also known as an air-bearing) caused by rotation of the disk 12 and the aerodynamic qualities of the slider portion of the head 20.
It is important that the flying height is maintained when the head is flying over portions of the disk surface that contain data. For example, if the head flies at too low a flying height, it is more likely to come into contact with the magnetic disk, which could potentially cause stored data to be lost. As another example, if the head flies too low, a particle resting on the disk surface may become attached to the head, which may cause the aerodynamic characteristics of the head to change. On the other hand, there are advantages to providing a relatively low flying height, especially because low flying heights are better suited for providing relatively small representations of binary digits (bits) and, thus, are associated with high data density. Accordingly, the “nominal” flying height that is employed, for a given disk, during normal operations (i.e. for reading data from the disk and providing it to a computer or other host device, or writing data to the disk, as requested by the host device, so as to substantially avoid data loss or errors), is a compromise between factors that favor low flying height and the risks that are associated with low flying height.
One particular phenomenon which causes low flying heights is known as pole tip protrusion. This phenomenon is described in connection with FIGS. 2, 3 and 4.
FIG. 3 is a simplified, air-bearing surface view of a conventional head 20, which illustrates a write portion 110 of the head and a read portion 120 of the head. For clarity, the slider portion of the head 20 is not shown.
The write portion 110 includes a write pole 130 and a return 135. The read portion includes a magneto-resistive (MR) element 140, along with first and second shields 142, 144. The direction of disk rotation is shown by arrow 150 in FIG. 3, such that the write pole 130 follows the MR read element 140.
FIG. 4 is a simplified, cross-sectional, side view of the conventional head 20 of FIG. 3. In addition to the elements shown in FIG. 3, FIG. 4 illustrates a write coil 155, a write gap 160 and a read gap 165.
As part of the writing process, a (typically variable) current is supplied to the coil 155 to induce magnetic flux across the write gap 160. The direction of the variable current defines the direction in which the magnetic flux will be oriented across the write gap 160. In some simple recording systems, flux polarized in one direction across the write gap 160 will record a binary “one” on the magnetic media while flux polarized in the opposite direction will record a binary “zero.” In most recording systems, a change in the direction that the flux travels across the gap 160 is interpreted as a “one” while the lack of a change is interpreted as a “zero.” As the disk 12 (shown in FIG. 1) travels under the write portion of the head 20, a series of digital “ones” and “zeros” can be written onto the disk surface 42.
During the read process, the first and second shields 142, 144 define a read gap 165 which serves to focus the flux for a particular magnetic polarity transition onto the read element 140 by shielding the read element 140 from other sources of magnetic flux. In other words, extraneous magnetic flux is filtered away from the read element 140 by the shields 142, 144. The MR read element 140 generates a signal in response to the changing magnetic field, which corresponds to a previously recorded data bit, as magnetic polarity transitions in the magnetic media pass underneath it.
Referring still to FIGS. 2, 3 and 4, the write portion 110 and the read portion 120 of the head 20 are located near the trailing edge of the head 20 (i.e., that portion of the head 20 that is closest to the disk surface 42). More specifically, since the write portion 110 trails the read portion 120 and since the head 20 is pitched (see FIG. 2) relative to the disk surface 42, it is the write portion 110 of the head 20 (specifically, the write pole 130) that is closest to the disk surface 42. In addition, in conventional heads, the write pole 130, return 135, first shield 142, MR element 140 and second shield 144 all have a surface (i.e., their respective surfaces which face the disk surface) which share a common plane 175.
It should be noted that most disk drives store information on disks using longitudinal recording techniques, as opposed to perpendicular recording techniques. However, the configuration of heads associated with longitudinal recording techniques may be very similar to the head shown in FIG. 4, such that its write pole, return, first shield, read element and second shield all lie in the same plane.
Although manufacture, distribution and use of disk drives follow a number of models, in at least some cases, following assembly of a disk drive, one or more testing procedures are performed. Often, testing is provided which is intended to identify, before they are distributed to users, any disk drives which may exhibit performance or reliability issues. In addition to reliability/performance testing, environmental testing may be performed. In some situations, environmental testing includes measuring and/or storing data related to how certain aspects of the disk drive react to various temperatures, pressures or other environmental factors. For example, environmental testing may be used to store information to control the magnitude of write head current as a function of ambient temperature (e.g., since a higher write current may be needed before the disk drive has “warmed up”).
Other operations may be performed prior to normal use of the disk drive (i.e. prior to use for reading and/or writing data sent to, or received from, the computer or other host device). In one such operation, the disk is provided with servo information such as sector markers or identifiers and track markers or identifiers. Servo information is generally distinguishable from data, at least, because location information is typically used for purposes of positioning the read/write heads (typically, internally to the disk drive) while data is received from or sent to the computer or other host device. The servo information is typically provided on the disk prior to any normal use of the disk. Further, as a general rule, the location information, once it is provided on the disk, is not, thereafter, altered or erased while, in most disk drives, data can be erased or written-over (although in some applications, some or all portions of disks may be designated as “read-only”).
The general trend in data storage, including disk drives, has been for increasingly higher data density on the medium. Higher densities permit not only construction of a physically smaller data storage device, for a given capacity, but also can assist in enhancing performance (reducing seek times, and the like). As will be understood by those skilled in the art, increases in data density are often associated with reductions in the distance between the read/write head and the medium (reduction in “flying height”). Among the technical difficulties encountered when attempting to reduce flying height, are those associated with the pole tip protrusion (PTP) phenomenon. When a write current is introduced into the write coil, the write current causes an amount of heating of the read/write head or arm tip, and the tip thermally expands. Accordingly, the tip is brought even closer to the disk surface 42. This phenomenon is known as pole tip protrusion and must be taken into consideration when designing heads. Failure to accommodate for pole tip protrusion can result in serious consequences, including data loss due to the tip contacting the disk and destroying information stored on the disk surface 42.
Among the parameters which affect the PTP phenomenon are the magnitude of the write current (higher magnitudes are associated with greater PTP), the duration of the write current (greater duration is associated with greater PTP), and the pattern of the write current (a varying pattern is associated with greater PTP, as opposed to, e.g. a D.C. write current).
In general it is a goal of disk drives to reliably read, from a disk, data which has been stored thereon. In the course of normal read operations, it occasionally happens that a read error is detected, i.e., it is determined that an attempt to read data from the disk does not result in a successful read of all target data. A number of conditions can be used to detect the occurrence of a read error including, e.g., the failure of a phase lock loop (PLL) system to achieve a lock condition with respect to a signal which is intended to be the data signal. Without wishing to be bound by any theory, it is believed that any of a number of conditions (or combinations thereof) can be responsible for a read error including, e.g., data being positioned (radially) off-track, data bits which vary, in their longitudinal (track-wise) position from the expected position, and/or ferromagnetic domains or transitions (which are typically used to represent data bits) which have a size or intensity less than the nominal or expected size or intensity.
When a read error occurs, previous approaches have typically included procedures intended to recover some or all of the non-read data. In some cases, data is encoded in a fashion such that if a sufficient portion of a data block is read, it is possible to reconstruct some or all of the data missing from a block. Typically, a system is also provided to attempt reading the data under various altered conditions such as using a “micro-jog” to position the read head incremental and small amounts off-track, relaxing phase lock constraints, varying bias error levels, and the like (or combinations thereof). In some previous devices, normal error recovery included buffering and erasing off-track (e.g., to try to remove coherent noise). Nevertheless, it is not unheard of that such previous, or “normal,” read error recovery attempts prove fully or partially unsuccessful.
Accordingly, it would be useful to provide different and/or additional read error recovery techniques for recovering data in response to a read error event. It would be especially useful to provide such different or additional techniques in a fashion which can be readily or inexpensively implemented, such as being capable of implementation with little or no additional modification of hardware, preferably such that the techniques can be implemented substantially by modification of software or firmware, alone (potentially permitting implementation during upgrading or repair of existing units, as well as in new units).